Wind power is considered one of the cleanest, most environmentally friendly energy sources presently available, and wind turbines have gained increased attention in this regard. A modern wind turbine typically includes a tower, a generator, a gearbox, a nacelle, and a rotor. The rotor typically includes a rotatable hub having one or more rotor blades attached thereto. A pitch bearing is typically configured operably between the hub and a blade root of the rotor blade to allow for rotation about a pitch axis. The rotor blades capture kinetic energy of wind using known airfoil principles. The rotor blades transmit the kinetic energy in the form of rotational energy so as to turn a shaft coupling the rotor blades to a gearbox, or if a gearbox is not used, directly to the generator. The generator then converts the mechanical energy to electrical energy that may be deployed to a utility grid.
A plurality of wind turbines are commonly used in conjunction with one another to generate electricity and are commonly referred to as a “wind farm.” Wind turbines on a wind farm typically include their own meteorological sensors that perform, for example, temperature, wind speed, wind direction, barometric pressure, and/or air density measurements. In addition, a separate meteorological mast or tower (“met mast”) having higher quality meteorological instruments that can provide more accurate measurements at one point in the farm is commonly provided. The correlation of meteorological data with power output allows the empirical determination of a “power curve” for the individual wind turbines.
For example, FIG. 1 illustrates a typical operating power curve 100 for a wind turbine. As shown, a typical wind turbine operates along an operating line 102 from a point “1” where wind speed is zero through points 2-3-4-5 (also known as the variable wind speed or knee region) to reach a rated power level 104 at point “5.” After reaching the rated power level 104, additional wind speed does not result in additional turbine power output.
Generally, it is important to optimize the operation of the wind turbine, including blade energy capture, to reduce the cost of the energy produced. To this end, each turbine includes a control module, which attempts to maximize power output of the turbine in the face of varying wind and grid conditions, while satisfying constraints like sub-system ratings and component loads. Wind turbine loads are dependent on the wind speed, tip speed ratio (TSR), and/or pitch setting of the blade. TSR, as is understood by those of ordinary skill in the art, is the ratio of the tangential velocity of the blade tip to the actual wind speed. Pitch settings of the blades (i.e., the angle of attack of the airfoil shaped blade), provides one of the parameters utilized in wind turbine control. Based on the determined maximum power output, the control module controls the operation of various turbine components, such as the generator/power converter, the pitch system, the brakes, and the yaw mechanism to reach the maximum power efficiency.
For example, wind turbine controllers are configured to adjust the rotational speed of the hub around which the blades rotate, i.e., the rotational speed, by adjusting the blade pitch in a manner that provides increased or decreased energy transfer from the wind, which accordingly is expected to adjust the rotor speed. As such, wind turbines are typically designed for a rated wind speed at which maximum thrust and maximum power generation occur.
Generally, the farm controller sends a fixed TSR command to each of the turbines in the wind farm to control rotor speed. As shown in FIG. 2, a relationship exists between rotor speed and torque as illustrated by torque-speed curve 200. The torque-speed curve 200 illustrates differing operating curves where curve 202 extending along points 1-2-5-6 represents a low torque demand design while curve 204 extending along points 1-3-4-5-6 represents a high torque demand design. As shown, the low torque demand curve 202 will reach a rated rotor speed 206 at point “2” where speed clipping will be observed prior to such turbine reaching its rated power at point “6.” The high torque demand curve 204, on the other hand, will reach its rated torque at point “4” (i.e. the point of torque saturation) and experience torque clipping prior to reaching its rated power at point “5.” Thus, some wind turbines experience a loss of power in the variable wind speed region due to torque saturation occurring earlier than the maximum rotor speed.
Accordingly, improved systems and methods for controlling wind turbines that address the aforementioned issues are desired in the art. In particular, systems and methods for controlling wind turbines using variable tip-speed-ratio control would be advantageous.